Bulk Transport


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•  The fluidity of membranes allows materials to be taken into cells by endocytosis or released by exocytosis 

The membrane is principally held together by weak hydrophobic associations between the fatty acid tails of phospholipids

This weak association allows for membrane fluidity and flexibility, as the phospholipids can move around to some extent

This allows for the spontaneous breaking and reforming of the bilayer, allowing larger materials to enter or leave the cell without having to cross the membrane (this is an active process and requires ATP hydrolysis)



The process by which large substances (or bulk amounts of smaller substances) enter the cell without crossing the membrane

  • An invagination of the membrane forms a flask-like depression which envelopes the extracellular material
  • The invagination is then sealed off to form an intracellular vesicle containing the material

There are two main types of endocytosis:

  • Phagocytosis – The process by which solid substances are ingested (usually to be transported to the lysosome)
  • Pinocytosis – The process by which liquids / dissolved substances are ingested (allows faster entry than via protein channels) 

Endocytosis via an Electron Microscope

endocytosis EM


The process by which large substances (or bulk amounts of small substances) exit the cell without crossing the membrane

  • Vesicles (typically derived from the Golgi) fuse with the plasma membrane, expelling their contents into the extracellular environment
  • The process of exocytosis adds vesicular phospholipids to the cell membrane, replacing those lost when vesicles are formed via endocytosis

Process of Exocytosis